I would always nurse in the AM before I left (I would let baby sleep till the last minute until I had just enough time to nurse and leave, Daddy took him to daycare). Then I would have my 2 15 minute breaks....one I would take probably 10ish , the other like 2-3 ish. I would also pump while I was eating lunch...I would get my food all ready attach pump, then I could kinda lean in towards the table to help stablize while I was eating. They have hands free pumping bras now too. Then first thing when I got home we would nurse. As he got older, I first cut out the lunch time pumping. Then it got to the point where he didn't want to wake in the morning so we cut that out. Then I got more flexible about after work, we would run errands etc before going home and nursing. I was able to continue for 14 months, although not without a few battles with my milk supply. I always nursed him when I was home at night or weekends. When my supply got low I got serious about nutrition, drank alot of water and took fenugreek.
You are so lucky to have a private office, most women struggle to find a place. I did sometimes have to take slightly more than 15 minutes but my employer never gave me a hard time about it. I would also take a picture with me and close my eyes and just imagine that I was cuddled up with my baby and try to remind myself of all the sensory feelings that went along with it and I would find that after a few min of doing that I woudl feel much more relaxed and the milk would be coming in force.
I too used a Medela pump in style and they are really the best ones out there. I have heard the Ameda works well too. You can also rent hospital grade pumps but they are like $30 a month, for about $300 you can have your very own that is almost as good. I was able to keep my milk in the communal fridge at work, I just tucked it in the back behind stuff people didn't use often. You can also keep it in a cooler with a cold pack if you don't have that. I used Playtex bottles and they make an adapter set that lets you pump right into the drop in liners so that was very helpful. I kept some dish soap and papertowels at work to clean up and some of the quick clean wipes for those times when I was rushed.
Good luck! It's hard work but totally do-able!